The invention relates to a method for determining movement data representing the movement of a machining tool of an optical lens 3D machining device for machining a surface of an optical lens.
The fabrication of an ophthalmic lens generally includes a first phase during which a blank is produced by molding and/or machining having an edge delimited by a front face and a rear face, and a second phase during which the blank is trimmed, i.e. the edge of the ophthalmic lens is machined to change it to a shape adapted for insertion in a given eyeglass frame.
During the first phase, correction properties corresponding to the prescription of the future wearer are conferred on the ophthalmic lens by the shape and the relative dispositions of the front and rear faces (the rear face being that which is turned towards the eye of the wearer of the correcting eyeglasses).
Some ophthalmic lenses, in particular so-called “progressive” lenses for correcting presbyopia, have a front face or a rear face that is asymmetrical with respect to the longitudinal axis of the cylinder formed by the edge of the untrimmed lens.
If a face of the lens is symmetrical with respect to that longitudinal axis, that face can be machined on the blank by making use of a standard turning process, the blank being driven in rotation about a rotation axis while a machining tool comes into contact with the lens to machine that symmetrical face.
On the other hand, if an asymmetrical face must be produced, the standard turning processes can no longer be employed in that they enable the machining only of shapes that are symmetrical with respect to the rotation axis of the part.
One solution for machining asymmetrical surfaces consists in making use of a method of machining a face of an ophthalmic lens including a machining stage during which the position of the machining tool is synchronized with the angular position of the ophthalmic lens driven in rotation about a rotation axis transverse to the face, so as to machine on the face a surface that is asymmetrical with respect to the rotation axis of the ophthalmic lens.
FIGS. 1 and 2 show the shape of a progressive ophthalmic lens 1. The view from above in FIG. 2 shows that this lens 1 has a circular contour. That circular contour is machined to correspond to the contour of a chosen spectacle frame.
FIG. 1 shows a typical profile of a progressive ophthalmic lens 1. The progressive ophthalmic lens 1 has a rear face 2 the curvature whereof is regular and a front face 3 the curvature whereof is greatly accentuated in a particular area of the progressive ophthalmic lens 1.
The progressive ophthalmic lens 1 therefore does not exhibit rotational symmetry with respect to the longitudinal axis 4 passing through the center of the circular contour of the progressive ophthalmic lens 1.
As illustrated on FIG. 3 an optical lens 1 is driven in rotation in the direction C about a rotation axis 10. A machining tool 14 mobile about a parallel translation axis 11 and a perpendicular translation axis 12 is driven in contact with the surface of the optical lens 1 to be machined.
The perpendicular axis 12 is the axis perpendicular to the rotation axis 10 defining with the rotation axis 10 a plan comprising the cutting edge 35 of the machining tool 14.
A turning device 16 is adapted to drive the optical lens 5 in rotation in the direction C. The position of the machining tool 14 at least along the parallel translation axis 11 is synchronized with the rotation.
The movement of the machining tool is usually determined according to the desired surface of the ophthalmic lens. Machining the surface of an ophthalmic lens according to such movement requires that the frequency of reversal of the translation movement of the machining tool 14 along the parallel axis be greater than the rotation frequency of the rotation axis.
Depending on the topology of the front face of the ophthalmic lens, it may be required that the frequency of reversal of the translation movement of the machining tool 14 along the perpendicular axis 12 be greater than the rotation frequency of the rotation axis.
For example, the machining of an asymmetric optical lens comprising a series of Fresnel zones requires that the frequency of reversal of the translation movement of the machining tool 14 along the perpendicular axis 12 be greater than the rotation frequency of the rotation axis.
Therefore, as explained above the machining of such optical lens requires the use of 3D machining devices having the frequency of reversal of the translation movement of the machining tool 14 along the parallel axis 11 and the perpendicular axis 12 be greater than the rotation frequency of the rotation axis.
Such 3D machining devices are very expensive and not very effective.